The present invention relates to plastic coils, and more particularly to a method for forming these plastic coils.
Plastic coils are well known through their several uses, for example to hingedly bind the pages of a book or booklet. The coils used this way are semi-rigid plastic wires forming a cylindroid helix of constant diameter and pitch. A particular advantage of this form of plastic coil binding element is that it is not expensive to produce, while being sturdy, offering a good aesthetic appearance and allowing an efficient and easy-to-use binding element for the book pages bound in this way.
A known method of producing plastic coils comprises using a conveyor line for a thermoplastic wire in which the wire is heated so as to achieve a semi-viscous pliable state, and then fed into a helical channel in which its selected helical form is achieved by cooling the channel so that the thermoplastic wire will solidify and recover its semi-rigidity in the form of a coil shaped according to the helical channel of constant selected pitch and diameter. The cooling is achieved by any suitable means of cooling the thermally conducting channel, whereby the plastic wire will be cooled by heat transfer therethrough.
The main problem associated with this conventional method for forming plastic coils is that the coil discharged by the helical channel is imparted with a rotative movement about its longitudinal axis, in addition to the desired output discharge translational movement along this same longitudinal axis. Thus, the plastic coil wire cannot be wound around a spool for storing and carrying purposes, because of the coiled wire rotating on itself which would result in an undesirable twisting of the plastic coil on the spool. The usual way to obviate this problem is to cut the coiled wire at regular - and relatively short - lengths, for example at every three feet. This effectively prevents the coils from twisting on themselves, but prevents the coils from being stored, carried and sold as a long, unitary coil. Not only is it less interesting to sell the coils in short lenghts, but also to cut the plastic coil is likely to result in an important waste of plastic coil material. Indeed, if the length at which the coil is cut (e.g. three feet) is not a multiple of the effective used lengths of the coils (e.g. to bind books having a length of two feet), then the extra coil material (e.g. one foot long for each three feet coil) must be discarded and is wasted.
It is the gist of the present invention to provide a plastic coil and a method for forming such plastic coils with a coil forming apparatus which improves on the existing methods, and more particularly which allows the output of the coil forming apparatus to feed a continuous, non-rotating plastic coil.
The present invention relates to a method for forming a continuous plastic coil from a straight thermoplastic wire, comprising the steps of:
a) heating the thermoplastic wire until it achieves a semi-viscous malleable state;
b) injecting the heated wire with an injection nozzle into a helical channel formed by a rotating threaded mandrel axially extending into a stationary sleeve and being radially adjacent therewith, with the wire sliding along the channel in a helical travelling direction to an outlet opening;
c) cooling said channel during the injection of the wire therein, wherein said wire will effectively be formed into a plastic coil of a shape corresponding to said helical channel;
d) rotating in a common rotative movement the mandrel and the injection nozzle during the injection in a direction opposite the travelling direction and at a speed allowing the continuous plastic coil to be discharged at the outlet opening with an axial translational displacement, while being exempt of any rotational displacement about its longitudinal axis.
Preferably, the method further comprises the following step after step d):
e) winding around a spool the discharged continuous plastic coil in successive turns.
The invention also relates to a spooled continuous plastic coil obtained from the method described hereinabove, comprising a spool and an elongated continuous plastic coil defining a longitudinal axis, said plastic coil being wound at least several turns around said spool and being substantially free of any torsional forces relative to its longitudinal axis.